The present invention relates to a brake-pressure producing device employing a hydraulic boosting device which boosts leg-power exerted on a brake pedal to a predetermined value by pressurized fluid, and more particularly to a brake-pressure producing device used in a semi-full power brake system having two circuits one of which is supplied with master cylinder pressure developed in a master cylinder and the other one is supplied with power chamber pressure introduced in a power chamber of a hydraulic boosting device.
A hydraulic boosting device used in a brake-pressure producing device of a vehicle can provide sufficient braking force with small leg-power on the brake pedal, thereby ensuring positive operation of the brakes and reducing the driver's labor.
In such a hydraulic brake system, it is desired to shorten the stroke of a brake pedal in view of its responsibility and operation. As one of conventional hydraulic boosting devices which can shorten the stroke of a brake pedal, proposed in Japanese Unexamined Patent Publication No. 63-247155 is a hydraulic boosting device used in a semi-full power brake system having two circuits one of which is supplied with master cylinder pressure developed in a master cylinder and the other one is supplied with power chamber pressure introduced in a power chamber of a hydraulic boosting device.
FIG. 5 is a view showing the hydraulic boosting device disclosed in this publication. In this figure, mark "a" designates the hydraulic boosting device, "b" designates a hydraulic booster, "c" designates a power piston of the hydraulic booster b, "d" designates a power chamber of the hydraulic booster b, "e" designates a control valve of the hydraulic booster b, "f" designates a master cylinder, "g" designates a piston of the master cylinder f, "h" designates a fluid chamber of the master cylinder f, "i" designates a line for the one circuit which communicates with the fluid chamber h of the master cylinder f, "i.sub.1 " and "i.sub.2 "designate branch lines of the fluid line i for the one circuit, "j.sub.1 "and "j.sub.2 " designate wheel cylinders relating to the one circuit which are connected to the branch lines i.sub.i and i.sub.2, respectively, "k" designates a line for the other circuit which communicates with the power chamber d of the hydraulic booster b, "k.sub.1 " and "k.sub.2 " designate branch lines of the fluid line k for the other circuit, "m.sub.1 " and "m.sub.2 " designate wheel cylinders relating to the other circuit which are connected to the branch lines k.sub.1 and k.sub.2, respectively, "n" designates a reservoir, "p" designates a hydraulic pump, and "q" designates a brake pedal.
In the hydraulic boosting device a, in the inoperative state i.e. when the brake pedal q is not pedaled, the control valve e which is in the inoperative position shuts off the power chamber d from the hydraulic pump p and communicates the power chamber d with the reservoir n. Therefore, no fluid pressure is introduced from the hydraulic pump p to the power chamber d so that the hydraulic booster b does not operate and the master cylinder f also does not operate so as not to actuate brakes. As the brake pedal q is pedaled, the control valve e operates to shut off the power chamber d from the reservoir n and communicate the power chamber d with the hydraulic pump p. As a result of this, fluid pressure is introduced from the hydraulic pump p to the power chamber d so that the power piston c works and the hydraulic booster b outputs whereby the piston g of the master cylinder f moves so as to develop master cylinder pressure in the fluid chamber h.
The master cylinder pressure is introduced into the wheel cylinders j.sub.1 and j.sub.2 through the lines i, i.sub.1, and i.sub.2 so as to actuate the brakes relating to the one circuit. The fluid pressure introduced into the power chamber d from the hydraulic pump p is further introduced into the wheel cylinders m.sub.1 and m.sub.2 through the lines k, k.sub.1, and k.sub.2 so as to actuate the brakes relating to the other circuit.
By using such a hydraulic boosting device a, it is no longer necessary to use a tandem master cylinder for two circuits, thereby shortening the axial length of an assembly of the hydraulic booster b and the master cylinder f and thus shortening the stroke of the brake pedal q.
However, the conventional brake-pressure producing device a has a problem that there is difference between brake pressures relating to the one circuit and that relating to the other circuit because the diameter of the power piston c of the hydraulic booster b and the diameter of the piston g of the master cylinder f are set to be different from each other. Thus, because of the difference in the brake pressures, the braking forces for braking the vehicle are not properly distributed to the respective wheel cylinders so that optimal braking is not conducted. Particularly, when the wheel cylinder for the front-left wheel and the wheel cylinder for the rear-right wheel pertain to the one circuit and the wheel cylinder of the front-right wheel and the wheel cylinder of the rear-left wheel pertain to the other circuit, i.e. in a case of the so-called X-type piping, the braking forces even for both the front wheels are different so as not to provide balanced braking.